search filters: add escaping and more LDAP operators

[keaz]

/Users/kasunranasinghe/Projects/Rust/simple-ldap/src/filter.rs currently exposes limited operators and builds LDAP filter strings directly.

Problem

Only Eq/Contains/Like/And/Or/Not are available and there is no explicit filter-value escaping layer. This increases risk of malformed filters and injection-like misuse, and limits parity with LDAP filter DSLs.

Proposal

Expand filter support:

• Add LDAP-safe value escaping utility.
• Add operators including Presence, >=, <=, = substring pieces, and extensible match helpers.
• Optionally include a raw filter wrapper for advanced callers.

Acceptance criteria

• Add tests for escaped special characters in values: *, (, ), \\, ,, +, =, and spaces.
• Reject malformed values that would produce invalid syntax.
• Document escaping behavior in module docs and examples.

[Iizuki]

No one has requested anything of this sort. Can we close this?